For previous reading, please refer to Injection Molded Part Wall Thickness Design 1: Understand These 3 Principles to Avoid 2 Years of Det.
Those in product development know that two biggest concerns in plastic part mold making are shrinkage and warpage. Many engineers' first reaction is to adjust process parameters. However, problem may actually lie in wall thickness design step. Too thin a wall results in insufficient filling; too thick a wall leads to severe shrinkage marks. Even the best process cannot salvage a wrong material selection.

 

This article will directly explain wall thickness design logic of mainstream engineering plastics.
First Principles of Wall Thickness Design
Before discussing specific materials, let's establish a consensus—core of wall thickness design is finding a balance between manufacturability, cost, and performance.
Material properties determine where this balance point lies:
• Good flowability → thinner wall thickness
• High shrinkage rate → uniform wall thickness, avoiding abrupt changes
• High strength → thin walls + reinforcing ribs can replace thick walls
Remember this framework to understand "why each material is designed this way."
Here's a quick reference table for wall thickness:
Material Wall Thickness Quick Reference Table
Based on historical experience, this is an initial design reference; final verification requires CAE validation.

(I) General Engineering Plastics
These materials are widely used, their wall thickness design needs to strike a balance between performance, cost, and manufacturability.
ABS: A Balanced Choice, Facilitating Thin-Wall Design
ABS, due to its excellent overall performance and processability, has become one of preferred materials for shell-type parts.
�� Recommended Wall Thickness Values

�� Material Properties and Their Influence
Good Flowability: ABS typically has high flowability, which allows for relatively thin wall thicknesses (down to 0.75mm) and ability to handle certain complex structures.
Low Shrinkage: Its molding shrinkage is relatively low and stable (approximately 0.5%), which makes it more tolerant of wall thickness uniformity. Wall thickness variations can be relaxed to ≤25%, and severe shrinkage marks are less likely to occur.
Strength and Cost Balance: Good mechanical properties allow for a balance between performance and cost by using moderate wall thicknesses (e.g., 1.5-2.5mm) while meeting functional requirements, without excessively thickening for strength.
2. PC: High Strength, Low Flowability, Uniformity is Paramount
PC is known for its high strength, high toughness, and transparency, but it is sensitive to wall thickness design.
�� Recommended Wall Thickness Values

�� Influence of Material Properties
Poor Flowability: PC melt viscosity is high, resulting in poor flowability. This dictates that wall thickness cannot be designed too thin, otherwise incomplete filling (short shots) is highly likely. A wall thickness of 0.8mm or more is typically required.
High rigidity: Its inherent high strength allows for reinforcement through structural designs such as stiffeners, rather than simply increasing wall thickness.
Strict requirements for uniformity: Shrinkage and internal stress issues make wall thickness uniformity crucial. Uneven wall thickness easily leads to warping, shrinkage marks, and stress cracking. A smooth, gradual transition is essential (recommended ratio ≤ 3:1).
3. PP: High elasticity, prone to shrinkage; avoid thickness, strive for uniformity.
PP is lightweight, inexpensive, and fatigue-resistant, but its high crystallinity presents significant shrinkage challenges.
�� Recommended Wall Thickness Values

�� Material Properties and Their Influence
Extremely High Shrinkage: As a highly crystalline plastic, PP has a large molding shrinkage rate. This is the most critical factor affecting wall thickness design—"because it is relatively soft, and due to shrinkage issues, it cannot be too thick." Excessive wall thickness will exacerbate shrinkage marks, warping, and voids.
Design Core: While ensuring filling and necessary strength, the smallest and most uniform wall thickness should be used to control shrinkage defects. For areas requiring strength, reinforcing ribs should be used instead of increasing glue thickness.

 

(II) High-Performance Engineering Plastics
These materials are typically used in applications requiring higher mechanical properties, heat resistance, or dimensional stability.
1. PA (Nylon): Tough and Moisture-Absorbent, Caution Regarding Shrinkage
PA is wear-resistant and has good toughness, but its high shrinkage and moisture absorption are design challenges.
�� Recommended Wall Thickness Values

�� Influence of Material Properties
High Shrinkage: Similar to PP, PA has a high shrinkage rate, requiring wall thickness to be as uniform as possible, and a gradual transition (≤3:1) to minimize warpage.
Good fluidity: Excellent melt fluidity, which is beneficial for filling thin-walled and complex structures, allowing for relatively thin wall thicknesses.
Water absorption affects performance: Mechanical properties differ significantly between dry and wet states; wall thickness design must ensure strength requirements are met even under the most unfavorable conditions.
Common reinforcement and modification: Reinforcement is often achieved by adding glass fiber (GF), requiring adherence to specific design principles for glass fiber reinforced materials.
2. POM (Polyoxymethylene): High rigidity and wear resistance, narrow processing window.
POM offers dimensional stability, wear resistance, and high rigidity, but has a narrow processing temperature window.
�� Recommended Wall Thickness Values

�� Material Properties Influence
High shrinkage rate: Typically 1.5-2.0%, requiring uniform wall thickness to avoid abrupt changes and prevent shrinkage depressions.
Narrow processing window: Sensitive to temperature; thick walls prolong cooling time, increasing risk of material degradation. Therefore, an economical wall thickness is generally recommended between 1.0-2.5mm.
High rigidity: Allows for thinner wall thicknesses (e.g., 1.5mm) to bear loads in small to medium-sized parts. For high strength requirements, reinforcing ribs should be used instead of simply increasing thickness.
3. PBT: Fast crystallization, prone to warping
PBT has good electrical properties and high strength, but as a semi-crystalline material, warping is a major challenge.
�� Recommended Wall Thickness Values

�� Influence of Material Properties
High shrinkage rate, prone to warping: Poor dimensional stability, wall thickness uniformity must be emphasized, and transitions at changes should be gradual with a ratio of ≤3:1.
Fast crystallization, good flowability: Beneficial for shortening molding cycles, but flow length still needs to be checked during design.
Often reinforced with glass fiber: Using materials such as PBT-GF30 can reduce shrinkage rate, increase strength, and provide more flexibility in wall thickness design.
(III) Special Performance Plastics
Designed for transparent, high-flow, or special flame-retardant requirements.
1. PMMA (Acrylic): Transparent but brittle, strength and uniformity are key. High light transmittance is desired, but material brittleness is its Achilles' heel.
�� Recommended Wall Thickness Values

�� Material Properties Influence
Brittle: High mechanical strength but poor toughness, prone to cracking. This requires a sufficient safety thickness (typically ≥0.8mm) to compensate for insufficient toughness, as wall thickness cannot be too thin.
High Light Transmittance: Extremely sensitive to defects such as bubbles, shrinkage marks, and stress lines. Therefore, requirement for wall thickness uniformity is higher than for ordinary materials. Thickness difference should be controlled within 25% of basic wall thickness.
2. LCP: High flowability, capable of extremely thin walls
Liquid crystal polymers are known for their unique high flowability.
�� Recommended Wall Thickness
Recommended thickness: Approximately 1.5mm. Thinnest possible: 0.4mm.
�� Material Properties and Their Influence
Extremely high flowability: This is its core characteristic, enabling it to perfectly fill extremely thin-walled and structurally complex areas. Wall thickness can be designed to be thinner than conventional materials to achieve lightweighting and miniaturization.
(IV) Modified Materials and Special Considerations
When a material undergoes flame-retardant or reinforcing modifications, its wall thickness design logic needs to be adjusted accordingly.
1. Flame-retardant Plastics
Wall thickness adjustment principles
Rating and thickness must be correlated: UL flame-retardant rating of material must be evaluated and reported along with specific thickness value. Different thicknesses of same material may correspond to different ratings.
Safety-driven: First, determine required flame-retardant rating according to product safety specifications, then select a material grade that meets rating at that thickness based on designed wall thickness.
Performance trade-offs: Flame retardants typically reduce mechanical properties of base plastic and may affect flowability. This trade-off must be considered during design; sometimes, it is necessary to appropriately increase wall thickness or compensate for strength loss through structural reinforcement, and avoid excessively thin walls that lead to filling difficulties.
2. Glass Fiber Reinforced Plastics (e.g., PA-GF, PBT-GF)
Wall Thickness Adjustment Principles and Influence of Glass Fiber:
Core Challenge: Anisotropic Shrinkage: Glass fiber restricts shrinkage in flow direction, resulting in vertical shrinkage being much greater than in flow direction (up to 2-3 times), easily causing warping.
Wall Thickness Design Response:
Extreme Emphasis on Uniformity: Wall thickness uniformity is even more crucial than with unreinforced materials to minimize differential shrinkage and warping caused by different fiber orientations.
Smoother Transitions: Smooth, long-slope transitions are essential at wall thickness changes.
Decreased Flowability: Increased glass fiber content reduces melt flowability. Design must reference spiral flow length data for specific glass fiber content of material to ensure wall thickness matches flow path and avoid short shots.
"Replacing Thickness with Ribs": Utilizing increased stiffness of glass fiber, thin-walled reinforcing rib structures are more actively employed rather than increasing overall wall thickness to reduce weight and control deformation.

 

(V) Summary of General Material Wall Thickness Adjustment Strategies

There is no absolutely correct wall thickness value, only a value that is "more suitable for this product."
All recommended values are initial design references based on historical experience. Final wall thickness must be verified and optimized through CAE analysis (e.g., mold flow analysis, structural simulation) in conjunction with specific structure, dimensions, load, and selected material grade (especially exact performance data after modification). For glass fiber reinforced or flame-retardant modified materials, performance data sheet for specific model and thickness provided by material supplier must be used as the standard.
Material manuals provide a reference starting point; truly optimal solution requires consideration of: Specific product structure; Mold gating system design; Process parameter adjustment; CAE mold flow analysis verification